Spread spectrum clock generator

ABSTRACT

Disclosed is a spread spectrum clock generator which includes: a first delay control type oscillator that variably controls an oscillation period at a control period interval according to a control signal; a control circuit; a maximum modulation value determination circuit that determines a maximum modulation value from a predetermined value, a frequency control signal, and a given modulation degree setting signal; a modulation signal generation circuit that receives the maximum modulation value from the maximum modulation value determination circuit and generates a modulation control signal within the maximum modulation value; and a second delay control type oscillator that receives a value obtained by adding the modulation control signal from the modulation signal generation circuit to the frequency control signal as a control signal and variably controls the oscillation period of an output clock signal at the control period interval according to the control signal. The control circuit outputs the frequency control signal for variably controlling the oscillation period based on the result of phase comparison between a clock signal obtained by frequency dividing the output of the first delay control type oscillator and a frequency divided clock signal of a reference clock signal, performed by a phase comparator.

FIELD OF THE INVENTION

The present invention relates to a clock frequency spreading (SpreadSpectrum Clock Generation: also referred to as “SSCG”) device, forreducing EMI (Electromagnetic Interference) of a semiconductorintegrated circuit.

BACKGROUND OF THE INVENTION

With increased operation speed and improved functions of semiconductorintegrated circuits, so-called EMI countermeasure for controllingradiated noise to be within a regulated value have especially becomedifficult. Thus, it sometimes happens that even if EMI countermeasurecomponents, such as an EMI filter or a decoupling capacitor, are mountedon a printed circuit board, the regulated value cannot be cleared.

Further, the mounting of the EMI countermeasure components on a printedcircuit board may result in an increase in the occupied area thereof andthe cost. In a situation where a lot of the semiconductor integratedcircuits are synchronized with a clock signal, an SSCG technique formodulating a clock frequency to reduce a peak spectrum thereof isdrawing attention.

FIG. 9 is a diagram showing a configuration described in Patent Document1.

Reference numeral Y1 31 denotes a piezoelectric crystal used in anoscillation circuit 32, and generates a stable clock pulse train or anunmodulated clock signal. A first programmable counter 35 divides(frequency divides) the unmodulated clock signal by an integer number(M). A clock signal output from a voltage controlled oscillator 39 (VCO)for adjusting an oscillation frequency based on a voltage from a filter38 is supplied to a buffer 40. From the buffer 40, an output clock isoutput. The frequency of the clock signal is proportional to an inputvoltage from a phase comparator 37 and the filter 38. The clock signaloutput from the VCO 39 is supplied to a second programmable counter 42,where the clock signal from the VCO 39 is frequency divided by aninteger number (N), for supply to the phase comparator 37.

The phase comparator 37 and the filter 38 generate an analog signal thatis proportional to the phase in error between the first programmablecounter 35 and the second programmable counter 42, respectively. Theclock signal from the buffer is thus equal to the oscillator frequencytimes (N/M). Spread spectrum modulation is performed using spreadspectrum modulation means 41 that changes the M and the N as a functionof time. A third programmable counter 45 divides the output of anoscillating circuit 32 by an integer number (I) that sets the rate thatthe M and the N change or modulation frequency.

First and second look-up tables 46 and 47 are, respectively, thestabilized values of the M and the N that modulate the frequency of theoutput clock signal.

An up/down counter 49 is used to index successive entries in the look-uptables. A serial link 51 may be used to program different values in theprogrammable counters or look-up tables to modify modulationcharacteristics.

FIG. 10 is a diagram showing other configuration described in PatentDocument 1. The spread spectrum modulation is performed by a second VCO52 and an analog modulated signal, which is the output of a digital toanalog converter 83 (DAC).

In case of no modulation, the second VCO 52 creates a clock signalidentical to the first VCO 39. The second VCO 51 responds to the analogmodulation to thereby create the spread spectrum clock output signal.

The modulation is performed through the use of a ROM 82 that storesmodulation amplitude values that are fed into the digital to analogconverter 83. The count output of an up/down counter 84 is used as anaddress in the ROM 82. A third programmable counter 85 sets themodulation frequency.

[Patent Document 1]

JP Patent Kokai Publication No. JP-A-7-235862 (FIGS. 6 and 9)

SUMMARY OF THE DISCLOSURE

In conventional configurations described above, there is a problem thata desired degree of modulation cannot be obtained due to the followingreasons:

-   -   manufacturing variations in the semiconductor integrated        circuit; and    -   a use environment such as a supply voltage, ambient temperature,        and a frequency used.

When the degree of modulation is small, the effect of reducing the EMIwill be reduced. On the contrary, when the degree of modulation isgreat, the maximum frequency of the clock will be increased, so that thecircuit which supplies the clock may malfunction.

In the semiconductor integrated circuit, due to the manufacturingvariations, the supply voltage, and the temperature used, thecharacteristics of transistors, resistors, and capacitors thatconstitutes the semiconductor integrated circuit will vary. For thisreason, the characteristics of the filter and the VCO will vary.

The configuration in FIG. 9 generates a modulated clock by changing thevalues of the M and the N. In other words, by causing the frequency ofthe VCO to respond to different frequencies, the modulated clock signalis generated. FIG. 11 shows an example of frequency transition whendesign is performed so that frequency response characteristicsconstituted from characteristics of the filter and the VCO and thechange time of the M and the N are optimized.

As described before, when the characteristics of the filter and the VCOhave varied and then a frequency response speed is slow, a targetfrequency is switched before the frequency of the modulated clockbecomes the target frequency, as shown in FIG. 12. The degree offrequency modulation is thus reduced. The larger the degree of themodulation, the larger the effect of reducing the EMI tends to be. Thus,in the case of FIG. 12, the degree of the modulation becomes smallerthan is expected, so that the effect of reducing the EMI is reduced.

On the contrary, when the frequency response speed is fast, there aregenerated states in which the frequency of the modulated clock exceedsthe target frequency and states in which the frequency of the modulatedclock is stabilized at the target frequency. When a clock that exceedsthe operating frequency of the circuit that supplies the clock is fed, amalfunction may be caused. On the other hand, when the frequency remainsunchanged, the effect of spectrum spreading will be reduced, so that theeffect of reducing the EMI may decrease.

FIG. 14 is a graph showing spectrum images in the states shown in FIGS.11, 12, and 13. Assume that the frequency response characteristic is thesame, but the target frequency of the clock differs. Then, the samephenomenon as in the case where the frequency response characteristichas been varied will occur.

When the frequency of the clock is increased, the phenomenon comparableto the case where the frequency response characteristic is slow willoccur. When the frequency of the clock is reduced, the phenomenoncomparable to the case where the frequency response characteristic isfast will occur.

Further, in the case of the configuration in FIG. 10, the second VCO 52that inputs as a control voltage a value obtained by adding the analogmodulated signal from the digital-to-analog converter 83 to the outputvoltage of the filter 38 generates the modulated clock. Thus, the periodof the analog modulated signal and the amplitude of the analog modulatedsignal become factors for determining the period of the modulation andthe degree of the modulation, respectively.

In the case of the configuration in FIG. 10, the characteristics of theVCO vary depending on the manufacturing variations and changes in thesupply voltage and the temperature used, as described before. Examplesshowing the characteristics of the control voltage over the VCO and theoutput frequency of the VCO under specific two conditions are shown inFIG. 15.

When an arbitrary frequency is output from the VCO, the control voltage(output voltage from the filter 38) differs depending on themanufacturing variations and conditions used. Thus, in the case wherethe analog modulated signal is added to the output voltage from thefilter 38 to generate a spread spectrum clock, even if the amplitude ofthe analog modulated signal is constant, the output voltage from thefilter 38 is different. Thus, the degree of modulation does not becomeconstant.

Further, when the target frequency is different as well, the samephenomenon will occur.

The invention disclosed in this application is generally configured asfollows:

A spread spectrum clock generation circuit according to one aspect ofthe present invention includes at least:

-   -   a delay control type oscillator for variably controlling an        oscillation period thereof at an equal interval, based on a        control signal;    -   a maximum modulation value determination circuit for determining        a maximum value of a modulated signal from a value obtained by        conversion of a minimum oscillation period of the delay control        type oscillator by a control period interval of the oscillation        period and an oscillation frequency control value for the delay        control type oscillator; and    -   a modulation signal generation circuit for receiving the maximum        modulation value from the maximum modulation value determination        circuit and generating a modulation control signal within the        maximum modulation value. The delay control type oscillator        receives a value obtained on adding the oscillation frequency        control value to the modulation control signal as the control        signal, and outputs a frequency modulated clock signal.

In the present invention, the value obtained by conversion of theminimum oscillation period of the delay control type oscillator by thecontrol period interval of the oscillation period is set to be constanteven when a delay value of an element is changed due to manufacturingvariations or a condition used, and the frequency modulated clock signalwith a degree of modulation maintained to be constant is output from thedelay control type oscillator.

A spread spectrum clock generation circuit according to another aspectof the present invention includes:

-   -   first and second delay control type oscillators each for        adjusting an oscillation period thereof at a predetermined        control period interval, according to a control signal;    -   a phase comparator for comparing a phase of a clock signal        obtained by frequency dividing an output clock signal of the        first delay control type oscillator with a phase of a clock        signal obtained by frequency dividing a reference clock signal;    -   a control circuit for generating a frequency control signal        based on a result of phase comparison by the phase comparator        and supplying the frequency control signal to the first delay        control type oscillator as the control signal;    -   a maximum modulation value determination circuit for determining        a maximum modulation value from a predetermined value, the        frequency control signal, and a given modulation degree setting        signal; and    -   a modulation signal generation circuit for receiving the maximum        modulation value from the maximum modulation value determination        circuit and generating a modulation control signal within the        maximum modulation value.

The second delay control type oscillator receives a value obtained byadding the modulation control signal from the modulation signalgeneration circuit to the frequency control signal from the controlcircuit as the control signal and outputs a frequency modulated clocksignal.

In the present invention, the predetermined value for the maximummodulation value determination circuit is set to a value obtained byconverting the minimum oscillation period of the second delay controltype oscillator by the control period interval of the oscillationperiod. Alternatively, the minimum value of the frequency control signalis set to a value obtained by converting the minimum oscillation periodof the second delay control type oscillator by the control periodinterval of the oscillation period.

A method of generating a spread spectrum clock signal according toanother aspect of the present invention includes the steps of:

-   -   determining the maximum value of a modulated signal from a value        obtained by conversion of the minimum oscillation period of a        delay control type oscillator by the control period interval of        an oscillation period and the oscillation frequency control        value of the delay control type oscillator, the delay control        type oscillator variably controlling the oscillation period        thereof at an equal interval based on a control signal;    -   generating a modulation control signal within the maximum        modulation value; and    -   supplying a value obtained by adding the oscillation frequency        control value to the modulation control signal to the delay        control type oscillator as the control signal, and outputting a        frequency modulated clock signal from the delay control type        oscillator.

In the present invention, the value obtained by conversion of theminimum oscillation period of said delay control type oscillator by thecontrol period interval of the oscillation period is preferably set tobe constant even when a delay value of an element is changed due tomanufacturing variations or a condition used; and the frequencymodulated clock signal with a degree of modulation maintained at adesired value is output from said delay control type oscillator.

In the present invention, by performing computation using the valueobtained by converting the minimum oscillation period of the delaycontrol type oscillator by the control interval of the oscillationperiod, the oscillation frequency control value, and a desiredmodulation degree, the maximum value of a modulated signal isdetermined. The delay control type oscillator adjusts an oscillationperiod thereof at an equal interval based on the control signal.

The change rate of the delay value of semiconductor devices is constanteven if the delay value of an element has been changed according to acondition used. Thus, the value obtained by converting the minimumoscillation period of the delay control type oscillator by the controlinterval of the oscillation period is made constant.

The oscillation period of the delay control type oscillator and theoscillation frequency control value plus the value obtained byconverting the minimum oscillation period by the control period intervalof the oscillation period are represented by a linear function thatcrosses the point of origin (0, 0). Accordingly, by determining themaximum value of the modulated signal using the “converted value plusthe control value”, the modulation degree becomes constant.

A delay-type phase adjusting circuit according to other aspect of thepresent invention includes:

-   -   a first variable delay circuit for receiving a reference clock        and adding a delay to the reference clock, for output;    -   a phase comparator for receiving an output of the first variable        delay circuit and the reference clock and detecting a phase        difference therebetween;    -   a control circuit for generating a control signal for variably        controlling a delay value of the first variable delay circuit        based on a result of phase comparison by the phase comparator;    -   a second variable delay circuit for receiving an input signal        and adding a delay to the input signal, for output; and    -   a computation circuit for receiving a predetermined value and        the control signal and variably controlling a delay value of the        second variable delay circuit.

A delay-type phase adjusting method according to yet another aspect ofthe present invention comprises the steps of:

-   -   detecting a phase difference between an output of a first        variable delay circuit and a reference clock by a phase        comparator, said first variable delay circuit inputting the        reference clock and adding a delay to the reference clock, for        output;    -   generating a control signal for variably controlling a delay        value of said first variable delay circuit by a control circuit,        said control circuit inputting a result of phase comparison by        said phase comparator;    -   variably controlling a delay value of a second variable delay        circuit by a computation circuit, said computation circuit        inputting a predetermined value and the control signal; and    -   inputting an input signal and adding the delay defined by said        computation circuit to the input signal, for output, by said        second variable delay circuit.

In the present invention, the predetermined value is set to a valueobtained by converting the minimum delay value of the variable delaycircuits by a delay control period interval.

In the present invention, the control signal outputting the minimumdelay value of the variable delay circuit is set to have a valueobtained by converting the minimum delay value of the variable delaycircuits by a delay control period interval.

The meritorious effects of the present invention are summarized asfollows.

According to the present invention, even if manufacturing variationsoccur or a supply voltage, a temperature used, or a frequency used isvaried, a frequency-modulated clock with a desired modulation degree canbe supplied.

According to the present invention, reduction in an EMI reduction effectcan be prevented.

According to the present invention, the modulation degree is made to bea desired value (a constant value) irrespective of the manufacturingvariations or an environment used. A malfunction of a circuit forsupplying a clock can be thereby prevented.

The present invention can help EMI standards to be cleared.

According to the present invention, a component used for EMI measurescan be reduced.

Still other effects and advantages of the present invention will becomereadily apparent to those skilled in this art from the followingdetailed description in conjunction with the accompanying drawingswherein only the preferred embodiments of the invention are shown anddescribed, simply by way of illustration of the best mode contemplatedof carrying out this invention. As will be realized, the invention iscapable of other and different embodiments, and its several details arecapable of modifications in various obvious respects, all withoutdeparting from the invention. Accordingly, the drawing and descriptionare to be regarded as illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration of a first embodiment of thepresent invention;

FIG. 2 is a graph showing relationships between an oscillation frequencycontrol value and an oscillation period in delay control typeoscillator;

FIG. 3 is a graph showing relationships between a converted value plusthe oscillation frequency control value and the oscillation period inthe delay control type oscillator;

FIGS. 4A and 4B include tables showing operation examples of controlsignals;

FIG. 5 is a graph showing modulation images of states A and B in FIG. 4;

FIG. 6 is a graph showing a spectrum of a step-type modulated clock;

FIG. 7 is a diagram showing a configuration of a fourth embodiment ofthe present invention;

FIG. 8 is a diagram showing a configuration of a fifth embodiment of thepresent invention;

FIG. 9 is a diagram showing an example of a configuration ofconventional SSCG;

FIG. 10 is a diagram showing an example of a configuration of theconventional SSCG;

FIG. 11 is a graph showing a frequency transition when a frequencyresponse is optimum;

FIG. 12 is a graph showing a frequency transition when the frequencyresponse is slow;

FIG. 13 is a graph showing a frequency transition when the frequencyresponse is fast;

FIG. 14 is a graph showing spectrums of states in FIGS. 11, 12, and 13;and

FIG. 15 is a graph showing a control voltage and output frequencycharacteristic of a VCO.

PREFERRED EMBODIMENTS OF THE INVENTION

Preferred embodiments of the present invention will be described. Theoperational principle of an embodiment of the present invention will beoutlined in the below. There are provided a first delay control typeoscillator (104) that variably controls an oscillation period at apredetermined control period interval according to a control signal, acontrol circuit (103), a maximum modulation value determination circuit(106) that determines a maximum modulation value (A) from apredetermined value, a frequency control signal (C), and a givenmodulation degree setting signal (MS), a modulation signal generationcircuit (107) that receives the maximum modulation value (A) from themaximum modulation value determination circuit (106) and generates amodulation control signal (α) within the maximum modulation value (A),and a second delay control type oscillator (108) that receives a valueobtained by adding the modulation control signal (α) from the modulationsignal generation circuit (107) to the frequency control signal (C) as acontrol signal and variably controls the oscillation period of an outputclock signal at the predetermined control period interval according tothe control signal. The control circuit (103) outputs the frequencycontrol signal (C) for variably controlling the oscillation period basedon the result of phase comparison performed by a phase comparator (102)between a clock signal (fo1) obtained by frequency dividing the outputclock signal of the first delay control type oscillator and a clocksignal (fi/M) obtained by frequency dividing a reference clock signal(fi).

The minimum oscillation period and the control period interval when thedelay control type oscillators (104, 108) are designed are indicated byT1 and D1, respectively. Then, the predetermined value (B) isconstituted from the value obtained by dividing the minimum oscillationperiod of the second delay control type oscillator by the control periodinterval. Then, the maximum modulation value determination circuit (106)outputs a value (B+C)×MS obtained by multiplying the predetermined value(B) added to the frequency control signal (C) by the modulation degreesetting signal (MS) as the maximum modulation value (indicated by “A”).The maximum period modulation of the output clock signal from the seconddelay control type oscillator (108) becomes (B+C)×MS×D0 (in which the D0is the control period interval of an actual device). The frequencycontrol signal (C) when the period of the output clock signal (fo1) ofthe first delay control type oscillator (104) becomes the period of thereference clock signal (fi) is given by T0+D0×C, in which the minimumoscillation period of the actual device is indicated by T0. Due to therelationship of the delay ratio of T0/D0=T1/D1, the modulationdegree=(B+C)×MS×D0/(T0+D0×C) becomes the MS, which is a constant valuewithout depending on manufacturing variations, a supply voltage, atemperature, and a frequency. A detailed description will be given belowin conjunction with embodiments.

FIG. 1 is a diagram showing a configuration of an embodiment of thepresent invention. Referring to FIG. 1, a spread spectrum clockgeneration circuit according to the present embodiment includes a firstprogrammable counter 101, the phase comparator 102, the oscillationfrequency control circuit 103, the first delay control type oscillator104, and a second programmable counter 105. The first programmablecounter 101 receives the reference clock fi and outputs the clock fi/Mobtained by frequency dividing the reference clock fi by M. The phasecomparator 102 receives the output fi/M of the first programmablecounter 101 at one input terminal thereof. The oscillation frequencycontrol circuit 103 receives a frequency error signal fgap, which is aresult of comparison by the phase comparator 102 and outputs thefrequency control signal C. The first delay control type oscillator 104receives the frequency control signal C from the oscillation frequencycontrol circuit 103 and outputs the oscillation clock fo1. The secondprogrammable counter 105 receives the output of the first delay controltype oscillator 104 and outputs a clock fo1/N obtained by frequencydividing the output of the first delay control type oscillator by N. Thephase comparator 102 receives the output of the second programmablecounter 105 at a second input terminal thereof and detects a phasedifference between the output fi/M of the first programmable counter 101and the output fo1/N of the second programmable counter 105. The spreadspectrum clock generation circuit further includes the maximummodulation value determination circuit 106, the modulation signalgeneration circuit 107, an adder 110, the second delay control typeoscillator 108, and a buffer 109. The maximum modulation valuedetermination circuit 106 receives the modulation degree setting signalMS and the frequency control signal C from the oscillation frequencycontrol circuit 103, for determination of a maximum modulation value.The modulation signal generation circuit 107 receives the maximummodulation value A output from the maximum modulation valuedetermination circuit 106 and outputs the modulation signal α. The adder110 adds the signal C from the oscillation frequency control circuit 103to the modulation signal α from the modulated signal generation circuit107. The second delay control type oscillator 108 receives an output C+αof the adder 110 and generates an oscillation clock fo2. The buffer 109receives the output of the second delay-type oscillation circuit 108 andoutputs a modulated clock fmo.

The phase comparator 102 and the oscillation frequency control circuit103 have a PLL configuration that performs a feedback operation suchthat the clock signal fi/M frequency divided by M by the firstprogrammable counter 101 has the same frequency as (is phasesynchronized to) the clock signal fo1/N frequency dividing the clocksignal fo1 output from the first delay control type oscillator 104 by Nusing the second programmable counter 105. Meanwhile, means for makingthe frequency of the oscillation clock fo1 constant is not limited tothis configuration.

The first delay control type oscillator 104 adjusts the period of theoscillation clock at an equal interval (referred to as a “control periodinterval”) according to the control signal C. The second delay controltype oscillator 108 adjusts the period of the oscillation clock at theequal interval (referred to as the “control period interval”) accordingto the control signal C+α.

In order to implement the degree of modulation specified from an outsideby the modulation degree setting signal MS, the maximum modulation valuedetermination circuit 106 determines the maximum value A from the valueof the oscillation frequency control signal C and the value obtained byconverting the minimum oscillation period of the second delay controltype oscillator 108 by the oscillation control period interval.

The modulation signal generation circuit 107 generates the modulationcontrol signal α of which the maximum value becomes the given A or less.As the modulation control signal α, the modulation signal generationcircuit 107 outputs 0, 1, 2, . . . , A−1, A, A−1, . . . , 2, 1, 0, −1,−2, . . . , −(A−1), −A, −(A−1), . . . , −2, −1, 0, 1, 2, . . . , forexample, in this stated order. That is, as the modulation control signalα, the modulation signal generation circuit 107 outputs 0 to A in a ¼modulation cycle, A to −A in a ¼ to ¾ modulation cycle, and −A to 0 in a¾ to one modulation cycle.

In the present embodiment, the second delay control type oscillator 108is controlled by the control signal C+α obtained by adding themodulation control signal α to the oscillation frequency control signalC, thereby obtaining the modulated clock signal fmo. When the α is zero,the frequency of the output clock of the first delay control typeoscillator 104 becomes the same value as the frequency of the outputclock of the second delay control type oscillator 108.

Period control is performed over the first delay control type oscillator104 and the second delay control type oscillator 108 at an equal controlperiod interval D0 from a minimum oscillation period T0. The first delaycontrol type oscillator 104 and the second delay control type oscillator108 are the oscillation circuits each of which adjusts the oscillationperiod thereof by adjusting a delay value in an oscillation loop basedon the input control signal. When the delay value is increased, theoscillation period becomes longer. When the delay value is reduced, theoscillation period becomes shorter. An arbitrary known configuration canbe employed as this delay control type oscillator. Though no particularlimitation is imposed, adjustment of the number of delay elements, forexample, can be performed as a configuration for adjusting the delayvalue of the oscillation loop. As an example, the delay control typeoscillator has the configuration in which the number of invertersconstituting the oscillation loop (in which the number of the invertersis an odd number) is adjusted and controlled based on the control signalin a ring oscillator where the output of the last stage of cascadedinverters constituted from the odd number of the inverters is feedbackconnected to the input of the first stage of the cascaded inverters. Inthis case, the control signal input to each delay control typeoscillator is associated with the number of stages of the periodintervals in FIGS. 4 and 5 that will be described later.

From the minimum oscillation period T1 and the control period intervalD1 at the time of designing, B is determined as follows:T 1/D 1=B  (1)

The maximum value A of the modulation control signal is determined fromthe following equation (2) with the modulation degree setting indicatedby the MS:A=(B+C)×MS  (2)

In order to obtain the period of the reference oscillation clock fo1,the frequency control signal C is determined at the oscillationfrequency control circuit 103 so that the frequency control signal Cbecomes T0+D0×C, with respect to the minimum oscillation period T0,control period interval D0, and the frequency control signal C in anactual device:T0+D0×C  (3)

As described before, the maximum period modulation is given by thefollowing equation (4) with the modulation degree setting indicated bythe MS and the control period interval indicated by the D0:(B+C)×MS×D0  (4)

The degree of modulation is given by the following equation (5):Degree of Modulation=[(B+C)×MS×D 0]/(T 0+D 0×C)  (5)

-   -   where    -   the B is given by T1/D1 (where the T1 indicates the minimum        oscillation period, and the D1 indicates the control period        interval),    -   the MS indicates the modulation degree setting,    -   the T0 indicates the minimum oscillation period,    -   the D0 indicates the control period interval, and    -   the C indicates the frequency control signal.

The change rate of the delay of the semiconductor devices caused by themanufacturing variations and the use environment thereof is constant.For this reason, the ratio T1/D1 between the minimum oscillation periodT1 and the control period interval D1 determined at the time ofdesigning and the ratio T0/D0 at the time of a different condition areconstant.

The degree of modulation is given by the following equations (7) whenthe following equation is substituted into the above equation (5):B=T 0/D 0  (6) $\begin{matrix}\begin{matrix}{{{Degree}\quad{of}\quad{Modulation}} = {\left\{ {\left( {B + C} \right) \times {MS}} \right\}/\left( {B + C} \right)}} \\{= {{MS}.}}\end{matrix} & (7)\end{matrix}$

As described above, the degree of modulation becomes the fixed value MS(modulation degree set value) that is independent of the manufacturingvariations, supply voltage, temperature used, and frequency.Accordingly, the modulation degree of the modulated clock signal is madeconstant.

The reason why the modulation degree becomes constant will be described,using other example. FIG. 2 shows relationships between the oscillationfrequency control value and the oscillation period in the delay controltype oscillators 104 and 108. Conversion of the minimum oscillationperiod by the control period interval is the same as giving an offset tothe oscillation frequency control value. When the oscillation frequencycontrol value (on a horizontal axis) is zero, the oscillation period (ona vertical axis) assumes a predetermined value.

Then, the relationship between the oscillation period and theoscillation frequency control value in which the minimum oscillationperiod (T) is converted by the control period interval (D) becomes alinear function that crosses the point of origin (0, 0) irrespective ofthe conditions used, as shown in FIG. 3. Thus, the maximum value of themodulation control signal calculated by multiplying the modulationdegree by:[Converted Value]+[Oscillation Frequency Control Value]  (8)has the same modulation degree with respect to the oscillation period aswell.

FIGS. 4A and 4B show numerical examples. FIG. 4A shows a case indicatedas follows:[Minimum Oscillation Period T 1]/[Control Period Interval D 1]=300(Fixed)  (9),and is an example where the modulation degree ±0.8% with a period of5,000 ps (picoseconds) is obtained.

In a state A in FIG. 4A, the minimum oscillation period T0 is set to3000 ps, and the control period interval D0 is set to 10 ps. Theconverted value of 300 is then calculated by the following equation:3000 ps/10 ps=300. When the value of the frequency control signal C is200, a synchronized state (in which the phase of fi/M is synchronizedwith the phase of fo1/N) is attained. Thus, the maximum value A of themodulation degree control signal becomes the value of four calculated bythe following equation: (300+200)×0.8%=4. The modulation period thusbecomes 40 ps using the equation of 4×10 ps=40 ps.

A state B in FIG. 4A shows a case where the delay value has become ahalf of that in the state A due to the influence of the manufacturingvariations and environmental fluctuations. The minimum oscillationperiod T0 is set to 1500 ps, the control period interval D0 is set to 5ps, and the converted value is set to 300, using the equation of 1500ps/5 ps=300. In the state B, when the value of the frequency controlsignal C is 700, the synchronization state is attained. Then, themaximum value A of the modulation degree control signal becomes eightusing the following equation: (300+700)×0.8%=8. The modulation periodthus becomes 40 ps obtained using the equation of 8×5 ps=40 ps. Themodulation degree in this case becomes ±0.8%, which is the same as theset value.

FIGS. 4A and 4B list the minimum oscillation period and the controlperiod interval. The maximum value of the modulation control signal,however, can be determined from the predetermined T1/D1 of 300 and thevalue of the frequency control signal C.

A state C in FIG. 4B shows a case where with the conditions of the stateA, a target period has become 7000 ps. In this case, too, the set degreeof modulation can be obtained. In the state C, when the value of thefrequency control signal C is 450, the synchronization state isattained, and the maximum value A of the modulation degree controlsignal becomes six using the equation of (300+450)×0.8%=6. Themodulation period becomes 60 ps using the equation of 6×10 ps=60 ps. Themodulation degree in this case becomes ±60 ps/7500 ps, which becomes the±0.8% that is the same as the set value.

FIG. 5 is a graph showing period transition of the states A and B inFIG. 4. Since the modulation is a digital period modulation, themodulation becomes a step-wise period modulation. Incidentally, thecontrol signal (C+α) input to the second delay control type oscillator108 in FIG. 1 defines the number of steps of the period intervals of theoscillation clock.

FIG. 6 is a graph showing a frequency spectrum obtained by applying anFFT (Fast Fourier Transform) to the step-wise period modulation in FIG.5. As shown in FIG. 6, it can be seen that even in the step-wisefrequency modulation, there is the effect of reducing a peak spectrum.

A second embodiment of the present invention will be described. In thefirst embodiment described before, the maximum modulation valuedetermination circuit 106 is configured to hold the value B obtained byconverting the minimum oscillation period of the second delay controltype oscillator 108 by the oscillation control period (oscillationfrequency control value). In the second embodiment, a minimumoscillation control signal value for the second delay control typeoscillator 108 is the value B obtained by converting the minimumoscillation period of the second delay control type oscillator 108 bythe oscillation control interval. In other words, the frequency controlsignal with the minimum value B is included.

The maximum modulation value determination circuit 106 in the firstembodiment has a computation circuit for the (B+C)×MS. In the secondembodiment, the maximum modulation value determination circuit 106becomes the computation circuit for C×MS. For this reason, the addercircuit for the (B+C) becomes unnecessary, thus reducing the size of thecircuit.

In the first embodiment described before, the operation of themodulation signal generation circuit 107 is generation of a signal of acenter-spread type. In the second embodiment, by repeating transition ofthe modulation control signal α in only a positive direction, such as 0,1, . . . , A−1, A, A−1, . . . 1, 0, 1 . . . , and the like, modulationof a down-spread type to a low frequency can be implemented.

As a third embodiment of the present invention, by repeating transitionof the modulation control signal α only in a negative direction, such as0, −1, . . . , −(A−1), −A, −(A−1), . . . −1, 0, −1 . . . , and the like,modulation of a up-spread type to a high frequency can be implemented.

Further, the transition of the modulation control signal α should bejust in the range of the maximum value A. The pattern of the transitionis not limited to those described above. Accommodation to varioussystems thereby becomes readily possible.

FIG. 7 is a diagram showing a configuration of a fourth embodiment ofthe present invention. Referring to FIG. 7, a third program counter 111is provided in a stage subsequent to the second delay control typeoscillator 108 in the first embodiment in FIG. 1. The third programmablecounter 111 frequency divides the output of the second delay controltype oscillator 108 by L. In the frequency division by L, the periodwhich provides a basis for the frequency division becomes L times.However, the period at the time of the modulation also becomes L times,so that the modulation degree remains unchanged.

In the fourth embodiment of the present invention, the frequency band ofan output modulated clock can be expanded without expanding thefrequency bands of the delay control type oscillators. Thus, an increasein the area is small because a low frequency band is handled.

Next, other embodiment of the present invention will be described. FIG.8 is a diagram showing a configuration of a fifth embodiment of thepresent invention. The fifth embodiment of the present invention shows acase where the invention has been applied to a delay-type phaseadjustment circuit (DLL: Delay Locked Loop).

In the delay-type phase adjustment circuit, a phase comparator 202 and adelay control circuit 203 perform an adjustment operation so that thedelay value of a first variable delay circuit 201 becomes the length ofone period of a received clock signal RCLK.

A second variable delay circuit 205 has the same characteristics as thefirst variable delay circuit 201. In order to output a desired phase setby a phase control signal OPH, a delay value is added to an input signalDQSI by a phase control signal DN. An arbitrary delay value can bethereby obtained.

The delay value of the second variable delay circuit 205 becomes thevalue delayed by one period of the RCLK when:DN=N  (10).

Through a computation using the ratio B(=T0/D0) between the minimum setdelay value T0 of a delay A of the variable delay circuit and the delaycontrol interval D0 under a certain condition, the value of a delaycontrol signal N, and the phase setting signal OPH, the phase outputsetting signal DN can be obtained.

When a phase of 90 degrees is necessary for the phase setting signalOPH, or when the OPH=90/360=¼ in a case where the delay of a quarter ofthe period is necessary, computation should be performed by using thefollowing equation:DN=(B+N)×OPH−B  (11)

The delay value necessary to obtain the delay of one period of the RCLKis given by:T0+D0×N  (12)

Assume that the control signal as follows is input to the secondvariable delay circuit 205:DN=(B+N)×¼−B  (13)

Then, the delay value of the second variable delay circuit 205 becomes:T0+D0×((B+N)×¼−B)  (14).

Assume that the following equation is substituted into the above formula(14):B=T 0/D 0  (15).

Then, the delay value of the second variable delay circuit 205 becomesas follows:¼×(T0+D0×N)  (16).

A delay of a quarter of one period of the RCLK can be thereby obtained.

As shown in the first embodiment, the change rate of a delay caused bythe manufacturing variations, supply voltage, and temperature used isconstant. Thus, the B should be determined under a certain condition inadvance.

When design is performed with the minimum delay control signal of thedelay adjustment circuit set as the B as in the fifth embodiment,computation can be performed using the following equation:DN=N×OPH  (17)

The variable delay circuit according to the present embodiment cannot beset to the minimum delay value or less, but there is no problem indesigning the variable delay circuit that can supply an output delayedby a phase of 90 degrees used in the case of a DDR (Double Data Rate)SDRAM device.

The foregoing description was given in conjunction with the embodimentsdescribed above. The present invention, however, is not limited to onlythe configurations of the embodiments described above, and of courseincludes various variations and modifications that could be made bythose skilled in the art within the scope of the present invention.

It should be noted that other objects, features and aspects of thepresent invention will become apparent in the entire disclosure and thatmodifications may be done without departing the gist and scope of thepresent invention as disclosed herein and claimed as appended herewith.

Also it should be noted that any combination of the disclosed and/orclaimed elements, matters and/or items may fall under the modificationsaforementioned.

1. A spread spectrum clock generation circuit comprising: a delaycontrol type oscillator for variably controlling an oscillation periodthereof at an equal interval, based on a control signal; a maximummodulation value determination circuit for determining a maximum valueof a modulated signal from a value obtained by conversion of a minimumoscillation period of said delay control type oscillator by a controlperiod interval of the oscillation period and an oscillation frequencycontrol value for said delay control type oscillator; and a modulationsignal generation circuit for receiving the maximum modulation valuefrom said maximum modulation value determination circuit and generatinga modulation control signal within the maximum modulation value; saiddelay control type oscillator receiving as the control signal a valueobtained on adding the oscillation frequency control value to themodulation control signal output and outputting a frequency modulatedclock signal.
 2. The spread spectrum clock generation circuit accordingto claim 1, wherein the value obtained by conversion of the minimumoscillation period of said delay control type oscillator by the controlperiod interval of the oscillation period is set to be constant evenwhen a delay value of an element is changed due to manufacturingvariations or a condition used; and the frequency modulated clock signalwith a degree of modulation maintained at a desired value is output fromsaid one of said delay control type oscillator.
 3. A spread spectrumclock generation circuit comprising: first and second delay control typeoscillators each for adjusting an oscillation period thereof at apredetermined control period interval, according to a control signal; aphase comparator for comparing a phase of a clock signal obtained byfrequency dividing an output clock signal of said first delay controltype oscillator with a phase of a clock signal obtained by frequencydividing a reference clock signal; an oscillation frequency controlcircuit, receiving a result of phase comparison by said phasecomparator, for generating a frequency control signal based on thereceived result of phase comparison and supplying the frequency controlsignal to said first delay control type oscillator as the controlsignal; a maximum modulation value determination circuit for determininga maximum modulation value from a predetermined value, the frequencycontrol signal, and a given modulation degree setting signal; and amodulation signal generation circuit for receiving the maximummodulation value from said maximum modulation value determinationcircuit and generating a modulation control signal within the maximummodulation value; said second delay control type oscillator receiving avalue obtained by adding the modulation control signal from saidmodulation signal generation circuit to the frequency control signalfrom said oscillation frequency control circuit as the control signaland outputting a frequency modulated clock signal.
 4. The spreadspectrum clock generation circuit according to claim 3, wherein thepredetermined value for said maximum modulation value determinationcircuit is set to a value obtained by converting a minimum oscillationperiod of one of the delay control type oscillators by the controlperiod interval of the oscillation period.
 5. The spread spectrum clockgeneration circuit according to claim 3, wherein a minimum value of thefrequency control signal is set to a value obtained by converting aminimum oscillation period of one of the delay control type oscillatorsby the control period interval of the oscillation period.
 6. The spreadspectrum clock generation circuit according to claim 3, wherein thepredetermined value (B) is set to a value obtained by dividing a minimumoscillation period of the second delay control type oscillator by thecontrol period interval; said maximum modulation determination circuitoutputs a value (B+C)×MS obtained by multiplying the modulation degreesetting signal for setting a modulation degree (MS) by a value obtainedby adding the predetermined value (B) to the frequency control signal(C), as the maximum modulation value (A); maximum period modulation ofthe output clock signal from said second delay control type oscillatoris given by (B+C)×MS×D0 (where D0 indicates the control periodinterval); the frequency control signal (C) when a phase of the outputclock signal of said first delay control type oscillator becomes a phaseof the reference clock signal is given by T0+D0×C, in which the minimumoscillation period is indicated by T0; and a modulation degree given bya formula (B+C)×MS×D0/(T0+D0×C) is the modulation degree set value (MS)due to B=T0/D0.
 7. The spread spectrum clock generation circuitaccording to claim 3, wherein said phase comparator inputs a firstfrequency-divided clock signal obtained by frequency dividing thereference clock signal by M (in which the M is a predetermined positiveinteger) by a first frequency divider; and a second frequency-dividedclock signal obtained by frequency dividing an output of said firstdelay control type oscillator by N (in which the N is a predeterminedpositive integer) by a second frequency divider is supplied to saidphase comparator.
 8. The spread spectrum clock generation circuitaccording to claim 3, further comprising: a first programmable counterfor receiving said reference clock signal and outputting a first clocksignal obtained by frequency dividing the reference clock by a firstfrequency division ratio; a second programmable counter for receiving anoutput from said first delay control type oscillator and supplying asecond clock signal obtained by frequency dividing the output from saidfirst delay control type oscillator by N by a second frequency divisionratio to a second input terminal of said phase comparator; an adder foradding the frequency control signal from said oscillation frequencycontrol circuit to the modulation control signal from said modulationsignal generation circuit; and a buffer for receiving an output of saidsecond delay control type oscillator and outputting a modulated clocksignal; wherein said phase comparator receives an output of said firstprogrammable counter at a first input terminal thereof; said maximummodulation value determination circuit receives the frequency controlsignal from said oscillation frequency control circuit and themodulation degree setting signal and determinines a maximum modulationvalue; and said modulation signal generation circuit receives themaximum modulation value output from said maximum modulation valuedetermination circuit and outputs a modulation control signal within themaximum modulation value.
 9. The spread spectrum clock generationcircuit according to claim 8, wherein the predetermined value for saidmaximum modulation determination circuit is set to a value obtained byconverting a minimum oscillation period of the second delay control typeoscillator by a control period interval.
 10. The spread spectrum clockgeneration circuit according to claim 8, wherein a minimum value of thefrequency control signal is set to a value obtained by converting aminimum oscillation period of the second delay control type oscillatorsby a control period interval.
 11. The spread spectrum clock generationcircuit according to claim 8, further comprising: a third programmablecounter for receiving the output of said second delay control typeoscillator and supplying to said buffer a signal obtained by frequencydividing the output of said second delay control type oscillator by athird frequency division ratio as an input.
 12. A semiconductorintegrated circuit device comprising the spread spectrum clockgeneration circuit according to claim
 1. 13. A delay-type phaseadjusting circuit comprising: a first variable delay circuit forreceiving a reference clock signal and adding a delay to the referenceclock signal, for output; a phase comparator for receiving an output ofsaid first variable delay circuit and the reference clock signal anddetecting a phase difference therebetween; a control circuit forgenerating a control signal for variably controlling a delay value ofsaid first variable delay circuit based on a result of phase comparisonby said phase comparator; a second variable delay circuit for receivingan input signal and adding a delay to the input signal, for output; anda computation circuit for receiving a predetermined value and thecontrol signal and variably controlling a delay value of said secondvariable delay circuit.
 14. The delay-type phase adjusting circuitaccording to claim 13, wherein the predetermined value is set to a valueobtained by converting a minimum delay value of the variable delaycircuits by a delay control period interval.
 15. The delay-type phaseadjusting circuit according to claim 13, wherein the control signaloutputting a minimum delay value of the variable delay circuit is set tohave a value obtained by converting a minimum delay value of thevariable delay circuits by a delay control period interval.
 16. Thedelay-type phase adjusting circuit according to claim 13, wherein basedon a result of comparison by said phase comparator, said control circuitperforms control so that a delay of the output of said first variabledelay circuit from the reference clock signal becomes one period of thereference clock signal.
 17. A semiconductor integrated circuit devicecomprising the delay-type phase adjusting circuit according to claim 13.